The present invention related to a method and apparatus for non-contact determination of the weight per unit area and/or the thickness of thin materials. The method involves directing an ultrasonic beam at the material and measuring the proportion of sound energy that passes through the material.
In the generic method, which is known from the German Patent No. 3 048 710, the length of time sound takes to travel directly from a transmitter to a receiver is first measured. The material being inspected is then introduced between the two components. To ensure that the receiver receives only direct sounds and no echoes, the transmitter transmits ultrasonic pulses, and the receiver turns on subsequent to a delay equal to the sound's travel time and off no later than twice that time. Gating is a technique that is in itself long known. Although it has significant advantages over conventional methods for the ultrasonic measurement of thickness, it does not solve all the problems involved. Since the aforesaid method is usually employed for inspecting paper money, the device can always be recalibrated between scanning one bill and the next.
When sheets of material are being inspected, on the other hand, it is not possible to recalibrate that often, and the sound travel time can be measured only prior to production. The distance between the transmitter and the receiver can vary as they expand and contract with fluctuations in temperature, as can the speed of sound in air, and the results will be affected accordingly. The results are even less reliable when the thickness of a sheet is to be measured from one edge to the other. In this event the transmitter and receiver shuttle back and forth across the sheet and there is absolutely no way of ensuring that the distance between them will remain absolutely constant.
The French Patent No. A 2 368 013 describes a system of off-surface detection of the thickness of materials. The transmitter and receiver are on the same side of the material. Phantom echoes can occur and extensively contaminate the results. The document discloses means of preventing the echoes from contaminating the results by appropriately controlling the transmitter such that no pulse is emitted until an interval longer than the sound's travel time plus the duration of the pulse elapses once the last echo has been received. This means that in the best possible case (when no echoes occur) there will be an interval of (travel time+pulse duration).times.2 between commencement of the transmission of one signal and commencement of a subsequent signal, so that not many measurements are available for reliable processing during the briefest possible measuring period.
A principal object of the present application accordingly is to provide both a method and device that overcome the aforesaid drawbacks and are practical for monitoring the thickness of sheets of material, even from one edge to the other, during production.
This object, as well as other objects which will become apparent in the discussion that follows, are attained in accordance with the present invention by providing a method and apparatus in which the transmitter and receiver are arranged on opposite sides of the thin material. The transmitter transmits ultrasound pulses spaced in time by pause intervals that are longer than the period of time sound takes to travel from the transmitter to the receiver. The output signal from the receiver is processed only when the amplitude of this signal exceeds a prescribed level. Also, only a portion of the output signal produced by the receiver in response to the receipt of each respective ultrasound pulse is processed. This portion of the output signal has a duration which is less than the time that sound takes to travel from the transmitter to the receiver.
The invention requires only a single and very coarse measurement of the distance between the transmitter and the receiver. Since measurement commences only at a specific amplitude, any change in the distance between the two components can be compensated for. All that is necessary is to displace the detection window. Whereas all the measurements are equal in duration, the moment when a particular measurement begins depends on the signal leaving the receiver. The beginning of a measurement can accordingly be scheduled for the same transition through zero independently of the distance between the transmitter and the receiver and hence of the sound's travel time. It accordingly becomes unnecessary to constantly recalibrate with the removal of material between the transmitter and receiver.
Processing involves digitally sampling the signal leaving the receiver in a practical way at an integral multiple of the sound's frequency. Amplitude and phase are obtained in accordance with the present invention by subjecting the signal to a Fourier transform. The detected parameters can then be compared with a reference curve or analytic function to express the area weight of the sheet.
The reference curve is established at regular intervals by measuring a reference of known thickness and density.
The process may also be calibrated by measuring the characteristics of a material having a known thickness and area weight, as a reference.
Since the process may be calibrated by using reference curves or analytic functions, or by employing a physical calibration reference, the output may generate data which corresponds to the thickness of the material or weight per unit area.
Even more precise results can be obtained in one version of the method by also compensating for any fluctuations occurring in the temperature of the air during the operation that can affect how long the sound takes to travel from the transmitter to the receiver while the distance remains absolutely constant. This approach requires another receiver at a different distance from the transmitter but with the signal leaving it being processed exactly like the signal leaving the first receiver.
Additional advantages of the present invention will be evident from the dependent claims, and various embodiments will now be specified with reference to the drawing.